Lazy load external images

[rilwis]

Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

I see lazy load was merged into version beta 16. However, I'm using version beta 17 and see it doesn't work for external images from imgur.com.

Reproducible demo

https://meta-box-docs-v2.netlify.app/custom-fields/

Steps to reproduce

Just visit a page with many images, the images below the fold are loaded immediately.

Expected behavior

The images below the fold should be lazy loaded.

Actual behavior

The images below the fold are loaded immediately.

Your environment

• Public source code: https://github.com/wpmetabox/docsv2/
• Public site URL: https://meta-box-docs-v2.netlify.app
• Docusaurus version used: 2.0.0-beta.17
• Environment name and version (e.g. Chrome 89, Node.js 16.4): Safari, Chrome and Firefox latest versions
• Operating system and version (e.g. Ubuntu 20.04.2 LTS): MacOS

Self-service

• I'd be willing to fix this bug myself.

[Josh-Cena]

I wouldn't call this a regression, because never have we transformed Markdown images that don't appear internal. I do believe this looks like a useful extension to make.